The genetic control of grain weight (GW) remains poorly understood. Quantitative trait loci (QTLs) determining the GW of rice were identified using a natural GW mutant, sgw. Using a segregating population derived from sgw (low GW) and cultivar 9311 (â9311â; indica, high GW), the chromosome segment associated with GW was detected on the short arm of chromosome 7. To validate and further refine the locus, QTL analysis based on F2 and F3 populations was conducted, and a single major QTL (designated as qsgw7) affecting the 1000-grain weight of paddy rice was identified on the short arm region of rice chromosome 7 between simple sequence repeat (SSR) markers RM21997 and RM22015, where 4 bacterial artificial chromosome clones, OJ1339_F05, P0506F02, P0011H09, and P0519E12, were present. Analysis of the near isogenic line for qsgw7 (NILqsgw7) showed that the grain length, width, and volume of paddy rice in NILqsgw7 were significantly lower than those in â9311â and that the 1000-grain weight, grain length, width, volume, and chalkiness of brown rice in NILqsgw7 were significantly lower than those in â9311â. These results suggested that the qsgw7 gene,which was identified in this study, may be a new GW-related QTL that could affect GW and grain shape, especially grain plumpness.